This invention relates generally to semiconductor integrated circuits and more particularly, it relates to a CMOS output driver circuit for reducing ground bounce.
Output drivers are commonly used within integrated circuits to couple data or control signals to external pins, thus coupling external devices to the integrated circuit. A CMOS output driver typically includes an output stage comprised of an NMOS and PMOS transistor. Referring to the output stage illustrated in FIG. 1, a PMOS transistor 110, the pull-up transistor, is connected between a high voltage source V.sub.cc (e.g., 5 volts) and an output terminal 112. When the pull-up transistor 110 is switched on by a low signal at its associated control gate, a logic high value is provided through the transistor's low impedance path to the output terminal 112 to drive signal-receiving circuitry 113. An NMOS transistor 116, the pull-down transistor, is connected between the output terminal 112 and ground. When the pull-down transistor 116 is switched on by a high signal at its associated control gate, the output of the terminal 112 is connected to ground through the transistor's low impedance path providing a low logic signal to the signal-receiving circuitry.
A common problem associated with output drivers is the occurrence of a phenomenon known as "ground bounce." Ground bounce occurs as a result of the parasitic inductance of the integrated circuit and packaging interconnect, and is a particular problem in integrated circuits that use high speed transistors. Stray inductances associated with the package interconnect are associated in part to the wire bonding, lead frame, and other sources. For purposes of illustration, the inductor 118 represents the stray inductances in the power path and the inductor 120 represents the stray inductances in the ground path. The capacitive reactance of the output driver load is represented in FIG. 1 by the capacitor 122.
Ground bounce occurs when the pull-down transistor switches from an off state to an on state. Referring to FIG. 1, when the pull-down transistor 116 is turned on the potential developed across the capacitor 122 is coupled by the transistor 116 across inductor 120. As a consequence, a transient (ground bounce) is generated across inductor 120. A surge of current flows from the output terminal 112 through the pull-down transistor 116 and through the parasitic inductance 120 to ground. Since the voltage across an inductance is directly proportional to the derivative of the current with respect to time, the voltage at the source of the pull-down transistor rises. This in turn decreases the gate-to-source voltage of the pull-down transistor. In the case where the pull-down transistor turns on very quickly, the derivative of current with respect to time is high and thus a relatively large decrease in gate-to-source voltage results. This decrease in gate-to-source voltage, caused by ground bounce, causes ringing and an increase in the overall propagation delay of the output buffer and can cause corruption of the internal logic. A symmetrical problem arises at transistor 110 when the driver output changes from ground to V.sub.cc. This effect is herein referred to as "V.sub.cc bounce."
Ground bounce and V.sub.cc bounce can cause problems in the quality of the signal driven by the driver circuits and can act to reduce the speed at which the circuit may effectively be operated. Bounce can also cause problems by creating noise in the power supply signal that is bussed throughout the circuit. This noise occurs as fluctuations in the voltage level V.sub.cc available on the power buss and can degrade overall circuit performance.
Modifications to the output driver have been made to decrease ground bounce. U.S. Pat. No. 5,124,579 to Naghshineh minimizes ground bounce by including a resistive means connected in series with certain gates of the transistors in the pull-up driver and pull-down driver to form RC delay circuits for limiting the rate of rise of the source voltage. U.S. Pat. No. 5,148,056 to Glass et al. minimizes ground bounce by coupling a feedback means to the output terminal of the output buffer. The feedback circuit provides a feedback voltage to the gate of the pull-down transistor to regulate the derivative of source current with respect to time.
From the above it is seen that an improved output driver is desired.